Biological Possibilities in the Next Ten Thousand Years 
The recognition of human physiological diversity may have 
enormous consequences. As soon as its genetical basis is under- 
stood large-scale negative eugenics will become possible. There 
may be no need to forbid marriage; few people will wish to 
marry a spouse with whom they share a recessive gene for micro- 
cephaly, congenital deafness, or cystic disease of the pancreas, 
so that a quarter of their children are expected to develop this 
condition. I cannot predict the later steps which will make 
positive eugenics possible, since we know the genetic basis of 
few desirable characters. I make some suggestions later. 
The second question, as to rare capacities, is more interesting 
if perhaps less important. I shall begin by giving an example of 
one. My late father was an examiner for certificates for would- 
be colliery managers. Among other things they had to detect 
and estimate small amounts of methane. When the wick of an 
oil safety lamp is turned down leaving a blue flame, the methane 
can be seen burning above it as a faint “‘cap’’, and its concen- 
tration, within a range below the explosive, can be estimated 
from the size of this cap. Most people can only see the cap in 
darkness after a few minutes’ adaptation. One day a candidate 
appeared who could do the estimation correctly by daylight. 
This capacity is certainly rare, but no one knows whether its 
frequency is one per thousand or one per million. It may have 
some drawbacks such as defective colour vision or a high demand 
for vitamin A. It is probably at least in part genetically deter- 
mined. 
Supernormal vision of any kind is certainly rare. Super- 
normal hearing is less so, but is only just beginning to be 
investigated. Supernormal smelling may be quite common. 
Supernormal muscular skill is highly prized when it is applied 
to certain sports, but no serious attempt has yet been made to 
measure it, or to determine how far it is genetically determined. 
Aptitude tests may eliminate the worst half or even three- 
quarters, but they do not pick out the one person per /akh (10°) 
who might become a really superb dentist or lens maker. 
One reason for this is that our consciousness is not closely 
connected with manual skill or muscular sense. Some would 
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